Electrochemical measurement of effective diffusivity in Hall‐Heroult electrolyte

Abstract

AbstractForced‐convection mass transfer at the metal/bath interface during aluminum reduction from cryolitic melts was studied under reproducible convective conditions. A film of molten aluminum on a rotating molybdenum cylinder was the cathode. Concentration overpotential measured as a function of rotation rate, current density, and bath composition was converted to concentration differences between the bulk and the metal surface. Chosen as the basis for calculation of a mass transfer coefficient was the concentration of aluminum fluoride given by: where the amounts of indicated compounds are in weight percent, IMP designates impurities, M is molecular weight, ρ is the density of the melt, and CR is the cryolite ratio, the ratio of moles NaF to moles AlF3. Agreement with a correlation for mass transfer to a rotating cylinder allowed the calculation of effective diffusivities for aluminum fluoride species, in alumina‐saturated melts, of: 11.1 ± 1.1 × 10−5 cm2/s at 1.8 CR; 11.4 ± 1.7 × 10−5 cm2/s at 2.3 CR; 5.4 ± 0.8 × 10−5 cm2/s at 3.0 CR; and 4.4 ± 0.9 × 10−5 cm2/s at 4.0 CR.